In GCIII I have completed 2 games using Iconian and Thalan. Because I always use the largest map, it took me over 400 hours.In GCII I completed 3 games (with largest map, also) using Altarian, Drengin and Yor about 7 years ago and I wrote the process as fiction stories.
So, now it is time to do some comparison between the 2 versions.
The graphic performance, the ship builder, the look of planets and universe, ... are greatly improved, althought these elements in GCII are still acceptable nowadays. But this is not the most important thing I want to say.
The game playing experience is greatly changed. Compared to the GCII, perhaps you can say that GCIII is more "realistic". The formation of the fleet greatly influence the battle outcomes; the size of the fleet now really matters (in GCII, the most effective way I found to minimize the battle casualty is to use just one powerful battleship without any smaller escorts. If you use escorts together with the battleship, the damage to the enemy is almost the same, but you will lose more escorts in vain). Now many small ships can really become a threat to a single battleship. The progress of the game is more smooth; the balance will changes steadily, and you can see that your empire grows more and more powerful and crash the opponent turn by turn.
But in GCII, there might be more fun, or "legendary" changes. I have several examples:
(1) I was playing Altarian and was attacked by Krynn armada (very large fleets of small ships). Accidently the dread lord appreared. I managed to conque one of the dread lord planet and stole Quantum Driver II technology, which is far more advanced than Altarian, Krynn or any other factions in the universe that time. By this powerful new weapon, I managed to hold all my planets before I can gether larger fleet to drive Krynn out of Altarian territory.
(2) I always put priority in warp speed and sensor range. And I will build such ships with minimum or no weapon and defense but with high speed and far sensor range. Their purpose is to attract the enemy fleet and lead them away from my planets, fleets and transport ships. If the espionage reached high level, the movement direction of enemy fleet can be predicted thus improve the efficiency of the distraction operation.
Because I always play suicidal difficulty level, all the AI are far more powerful than me in the early phase of time. Attract the main force of enemy fleet to stray away from their planets (and our planets), send in single fleet to avoid any unnecessary engagement with enemy resistance, and launch sudden attack with high speed transport out of enemy sensor rage, steal the desired technology from the opponent by load/save... This seems to be the only way to conque the first opponent, get some advantage on empire size and population and greatly reduce the technology gap in the first 100 turns. With this strategy, the week is possible to overwhelm the strong.
(3) The experience system of battleships. Accumulate experience to level up your battleships can increase its attack power and hit points, and more importantly, recover the HP to 50% in a blink of eye when level up. Normally when I started (or forced to face) the first war, I only have 3~4 strengthened capital ships/small fleets against hundreds of fleets. And we have 20+ planets to take, in no more than 30 turns. And my technology is inferior. So even if we won the battle, the capital ship took a lot of damage. And we do not time to repair them before we engage the next wave or attack the next planet... Imagine when your capital ship has lower than 10% HP and there are two more enemy fleets appear between you and the next defended enemy planet, and your capital ship rush into one enemy fleet desperately; then suddenly your ship and crew are leveled up and recover 50% HP, and without a hesitation they storm the planet and set out for the next planet deeper in the enemy territory , leaving the astonished enemy fleet behind their back. You'd think that the Captain of this ship must be someone like Kirk... and I feel urged to rename the capital ship to "Enterprise", or "Excelsior", or "Voyager". It is really exciting moment.
(4) Balance change in the mid-term, even late phase of the game. In GCII, even if I defeated the first opponent, there are still many unpredictable events can alter the balance and progress greatly. Sometimes, 3~4 factions (or all other factions) will declare war against one civilization. This could be bad or good, depending on whether you are the cursed one.
(5) When I played Drengin, a minority Akilian got hold of all advanced technolgy of terran. But I did not have any valuable things to trade with them. So I sent Kona (a planet near Drengin home world) as a gift to Akilian, but attacked and took it back immediately after that, and gave it to Akilian again. After a dozen times, Drengin stole all the important technology from Akilian in TWO TURNs！At the cost of scores of billion people and dozens of transport ship.
(6) Drengin has an ability that when a war breaks, free super dominator corvette will appear to support the warfare. They are free but if you have dozens of planets, the maintanance cost is still high. But their attack is really low... So I declared war with other civilizations repeatedly and got hundreds of them, then trade them with Terran for loads of advanced technology and economy treaty (AI in suicide levle has very powerful economy. 10% of their income is usually more than my income)...
Now in GCIII if you defeated the first opponent, all the progress can be predicted and all you have to do is to conque other planets one by one. Of course, there are also some new interesting tactics:
(1) Just as some mentioned in the forum, in the earlier phase, the carrier is used to provide renewable drones to take the damage from the enemy instead of battleship. Battleship is the main exporter of damage. But as the you develop more advanced weapon and defense techs, the carrier become more powerful and eventually can eliminate an entire fleet alone without a scrach.
(2) Use resorce to equip protoype weapons and engines, is an alternate before technology breakthrough.
In general, I think maybe GCIII can introduce more elements to change the balance greatly. Mercenary is a good try, but I have not experienced it.